The answer is a missing route in the VPC route table for the on-premises CIDR block pointing to the virtual private gateway. Even when troubleshooting VPN tunnels up but no traffic due to missing route, the VPN status showing 'available' confirms the IPsec tunnels and BGP sessions are established, but the VPC itself has no forwarding instruction for traffic destined to 172.16.0.0/16. Without that static or propagated route targeting the virtual private gateway, the VPC drops the packets at its edge, unable to send them through the VPN connection. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that tunnel health and routing are separate layers—a common trap is assuming an 'available' VPN means traffic flows. Remember the memory tip: "Tunnel up, route missing? Traffic stops at the gateway."
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Network Topology
A network engineer is troubleshooting a VPN connection between an on-premises network (172.16.0.0/16) and an AWS VPC (10.0.0.0/16). The VPN status is 'available' but traffic is not passing. The engineer runs the command shown in the exhibit. What is the most likely cause of the issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The VPC route table does not have a route for 172.16.0.0/16 pointing to the virtual private gateway.
The VPN status is 'available', which indicates that the VPN tunnels are established and the BGP sessions (if configured) are up. However, traffic still fails because the VPC route table lacks a route for the on-premises CIDR (172.16.0.0/16) pointing to the virtual private gateway (VGW). Without this route, the VPC does not know to send traffic destined for the on-premises network through the VPN connection, even though the tunnels themselves are operational.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The VPN tunnels are not in 'UP' state.
Why it's wrong here
The state is 'available', meaning tunnels are up.
✓
The VPC route table does not have a route for 172.16.0.0/16 pointing to the virtual private gateway.
Why this is correct
Without this route, VPC traffic to on-premises is dropped.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The tunnel inside CIDRs overlap with the VPC CIDR.
Why it's wrong here
169.254.x.x are link-local, not overlapping with 10.0.0.0/16.
✗
The BGP session is not established.
Why it's wrong here
Static routes are used; BGP may not be configured.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates see 'available' status and assume all components are working, but they overlook the critical requirement of a route in the VPC route table pointing to the virtual private gateway for the on-premises CIDR.
Detailed technical explanation
How to think about this question
In AWS Site-to-Site VPN, the VPN connection status reflects the health of the IPsec tunnels and BGP sessions, but traffic forwarding depends on route propagation or static routes in the VPC route table. Even with both tunnels 'UP' and BGP established, the VPC will drop traffic to the on-premises network if no route exists for the remote CIDR via the VGW. This is a common misconfiguration where the engineer focuses on tunnel status rather than the routing table, which is the final arbiter of packet forwarding in the VPC.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The VPC route table does not have a route for 172.16.0.0/16 pointing to the virtual private gateway. — The VPN status is 'available', which indicates that the VPN tunnels are established and the BGP sessions (if configured) are up. However, traffic still fails because the VPC route table lacks a route for the on-premises CIDR (172.16.0.0/16) pointing to the virtual private gateway (VGW). Without this route, the VPC does not know to send traffic destined for the on-premises network through the VPN connection, even though the tunnels themselves are operational.
What should I do if I get this ANS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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